Today most manufacturing processes involve the manufacture of more or less complicated structures from several parts of metallic material such as steel. To achieve a solid structure various welding techniques are used to join these parts together in a durable fashion. In the case of vehicle cages the durability of these cages are of paramount importance for the safety of vehicle operators and passengers e.g. in case of accidents such as collisions. The strength of the welds between these parts is critical for the durability of the structure, especially under load.
A commonly used process for enabling UHSS (Ultra High Strength Steel) involves hardening the parts by various means to achieve hot formed parts (U.S. Pat. No. 3,257,835, Method for hot forming metal) and subsequently pressing them to achieve the desired shape, as well as allowing them to cool. What follows are a variety of post-processing techniques aimed at achieving various qualities of the material. After said post-processing, said metallic parts are joined together by various welding techniques. Such welding techniques may include spot-welding, laser welding, gas metal arc welding (GMAW), etc. During such welding processes, heat added leads to a phase change from a martensite structure to contain also bainite, ferrite and/or perlite structure in the already hot formed parts. The bainite, ferrite and perlite structures have the effect of lowering the durability of the material; for example, the elongation (A80) of martensite is about 5%, whereas the same value for ferrite and perlite is greater than 15%. The proof stress (Rp0, 2) and tensile strength (Rm) of martensite compared to ferrite and perlite are 1050 MPa versus 450 MPa and 1500 MPa versus 600 MPa, respectively. These properties are of great importance for the ability of a material to provide an effective seal and withstand stress, which are critical aspects of durability. Especially affected are the heat-affected zones (HAZ). The HAZ are defined as the regions around a weld center susceptible to structural change due to the heating resulting from the welding, in the range of 3.5 mm to 7 mm from the weld center. Structural change in these areas may lead to the creation of metallurgical notches which are especially susceptible to cracks when loaded.